Some electrochemical devices depend on the flow of protons, or the flow of both protons and electrons, through a proton conducting material, such as a membrane. Accordingly, materials which conduct protons, or both protons and electrons, have applications as electrolytes or electrodes in a number of electrochemical devices including fuel cells, hydrogen pumps, supercapacitors, sensors, hydrogen separation membranes and membrane reactors.
One particularly important application for these materials is in fuel cells. Fuel cells are attractive alternatives to combustion engines for power generation, because of their higher efficiency and the lower level of pollutants produced from their operation. There are three common types of fuel cells relevant to this patent: 1) direct hydrogen/air fuel cells, in which hydrogen is stored and then delivered to the fuel cell as needed; 2) indirect hydrogen/air fuel cells, in which hydrogen is generated on site from a hydrocarbon fuel, cleaned of carbon monoxide, and subsequently fed to the fuel cell; and 3) direct alcohol fuel cells, such as methanol (“DMFC”), ethanol, isopropanol and the like, in which an alcohol/water solution is directly supplied to the fuel cell. An example of this later fuel cell was described, for example, in U.S. Pat. No. 5,559,638, the disclosure of which is incorporated herein by reference.
Regardless of the fuel cell design chosen, the operating efficiency of the device is partly limited by the efficiency of the electrolyte at transporting protons. Typically, perfluorinated sulphonic acid polymers, polyhydrocarbon sulfonic polymers, and composites thereof are used as electrolyte membrane materials for fuel cells. However, these conventional materials utilize hydronium ions (H3O+) to facilitate proton conduction. Accordingly, these materials must be hydrated, and a loss of water immediately results in degradation of the conductivity of the electrolyte and therefore the efficiency of the fuel cell. Moreover, this degradation is irreversible, i.e., a simple reintroduction of water to the system does not restore the conductivity of the electrolyte.
As a result, fuel cells utilizing these materials require peripheral systems to ensure water recirculation and temperature control to keep the water from evaporating. These peripheral systems increase the complexity and cost of these fuel cells, from the use of expensive noble catalysts (Pt) to temperature requirements that cannot exceed much above 100° C. As a result of these temperature limitations, the fuel cell catalysts and other systems cannot be operated to maximum efficiency. Higher temperatures can also reduce carbon monoxide poisoning of the fuel cell catalyst.
It has recently been shown that the solid acids such as CsHSO4 can be used as the electrolyte in fuel cells operated at temperatures of 140-160° C. (Haile, S. M.; Boysen, D. A.; Chisholm, C. R. I.; Merle, R. B. Nature 2001, 410, 910-913). Use of this material greatly simplifies fuel cell design relative to polymer electrolyte fuel cells because hydration of the electrolyte is not necessary and, because of the elevated temperature of operation, residual CO in the fuel stream can be better tolerated. The high conductivity of CsHSO4 and analogous materials results from a structural phase transition (referred to as a superprotonic phase transition) that occurs at 141° C. from an ordered structure, based on chains of SO4 groups linked by well-defined hydrogen bonds, to a disordered structure in which SO4 groups freely reorient and easily pass protons between one another. Across this transition, the protonic conductivity increases by 3 to 4 orders of magnitude from 10−6 Ω−1 cm−1 (phase II) to 10−3-10−2 Ω−1 cm−1 (phase I; Baranov, A. I.; Shuvalov, L. A.; Shchagina, N. M. JETP Lett. 1982, 36(11), 459-462). Thus, disorder in the crystal structure is a key prerequisite for high proton conductivity.
However, the lifetime of these sulfate and selenium based solid acids is insufficient for commercial applications (Merle, R. B.; Chisholm, C. R. I.; Boysen, D. A.; Haile, S. M. Energy & Fuels 2003, 17, 210-215). The poor lifetime of both CsHSO4 and CsHSeO4 under fuel cell operating conditions results from the reduction of sulfur and selenium by hydrogen in the presence of typical fuel cell catalysts, according to:2 CsHSO4+4H2→Cs2SO4+H2S+4H2O.2 CsHSeO4+4H2→Cs2SeO4+H2Se+4H2O.
Recently, it has been shown that CsH2PO4 has as superprotonic transition and is stable under fuel cell conditions (Boysen, D. A.; Uda, T.; Chisholm, C. R. I.; Haile, S. M. Science 2004, 303, 68-70). To date this is the only known solid acid compound meeting the necessary conditions of long term chemical stability for operation as a fuel cell electrolyte. However, the compound is water soluble and only becomes useful as an electrolyte above its superprotonic phase transition at 230° C. (Baranov, A. I.; Khiznichenko, V. P.; Shuvalov, L. A. Ferroelectrics 1989, 100, 135-141). Accordingly, a need exists for solid acid compounds with high proton conductivity over a large range of temperatures that are stable under fuel cell conditions.